1. Field of the Invention
The present invention relates to a method in a wireless communication system according to the preamble of the appended claim 1. The invention also relates to a wireless communication system according to the preamble of the appended claim 11. The invention also relates to a wireless terminal according to the preamble of the appended claim 21 and to an access point according to the preamble of the appended claim 22.
2. Brief Description of Related Developments
Communication systems intended for an office environment, so-called local area networks (LAN), are primarily implemented as wired systems. Thus, the connection between the terminals and the server is implemented either electrically by means of a cable or optically by means of an optical fibre. An advantage of such a fixed system is, for example, the possibility to achieve relatively high data transmission rates. A disadvantage of such a fixed communication system is that it is difficult to make changes, and the terminals must usually be placed relatively close to connection points intended for them, which affects the movability of the terminal. The implementation of such a wired local area network in an already existing building is not always possible, or it is expensive to install the cables afterwards. On the other hand, particularly in older buildings, there possibly already exists a communication cabling which is not necessarily suitable for fast data transmission.
There are various wireless communication systems under development for implementing local area networks. Several wireless communication systems are based on the use of radio signals in data transmission. One such communication system for a local area network under development, based on radio communication, is the so-called HIPERLAN (High PErformance Radio Local Area Network). Such a radio network is also called a broadband radio access network (BRAN).
In version 2 of the HIPERLAN communication system under development, the aim is to achieve a data transmission rate of even more than 30 Mbit/s, the maximum connection distance being some tens of metres. Such a system is suitable for use in the same building e.g. as an internal local area network for one office. There is also a so-called HIPERACCESS communication system under development, in which the aim is to achieve the same data transmission rate as in said HIPERLAN/2 communication system, but the aim is to achieve a connection distance of several hundreds of metres, wherein the HIPERACCESS system is suitable for use as a regional local area network for example in schools and larger building complexes.
In the data link layer DLC of the HIPERLAN/2 system used as an example, the MAC (Medium Access Control) frame structure is illustrated in a reduced manner in the appended FIG. 1b. The data frame FR consists of control fields C, such as RACH (Random Access Channel), BCCH (Broadcast Control Channel), and FCCH (Frame Control Channel), as well as of a data field D which comprises a certain number of time slots TS1, TS2, . . . , Tsn, in which it is possible to transmit actual payload information.
Each control field C as well as the packets to be transmitted in the time slots of the data field preferably contain error check data which is calculated by the access point AP1 transmitting the data frame and added into the control fields C of the data frame and the packets to be transmitted in the time slots TS1, TS2, . . . , TSn. This check data is preferably a checksum calculated on the basis of information contained in said field, such as CRC (Cyclic Redundancy Check). In the receiving wireless terminal MT1, it is possible to use the error check data to examine whether there were possible errors in the data transmission. The field C, D may also contain several items of such check data calculated from part of the information contained in the field. For example in the HIPERLAN/2 system, the FCCH control field consists of smaller information elements, check data being calculated for each of them. The number of these information elements can vary in each data frame. All data frames do not necessarily have an FCCH control field, wherein also the number of information elements is zero.
Data transmission in the HIPERLAN/2 system is based on time division multiple access TDMA, wherein there can be several simultaneous connections on the same channel, but each connection is allocated a separate time slot in said frame, wherein data is transmitted. Because the quantity of data to be transmitted is normally not constant in all the simultaneous connections, but it varies with time, a so-called adapting TDMA method is used, wherein the number of time slots to be allocated for each data transmission connection may vary from zero to a maximum, depending on the loading situation each time as well as on the data transmission capacity allocated for the connection.
For time division multiple access to work, the terminals coupled to the same node must be synchronized with each other and with the transmission of the node. This can be achieved for example in such a way that the receiver of the wireless terminal receives signals on a channel. If a signal is not detected on the channel, the receiver changes over to receive on another channel, until all the channels have been examined or a channel is found on which a signal transmitted by an access point is detected. By receiving and demodulating this signal, it is possible to determine the moment of transmission of the control channel BCCH of the access point in question and to synchronize the terminal on the basis thereof. In some cases, the terminal can detect the signal of more than one access point, wherein the terminal advantageously selects the access point which has the strongest signal in the receiver, and performs the synchronization with this access point.
When the terminal is synchronized with the access point, the terminal can initiate a connection set-up to couple to this access point. This can be conducted advantageously in such a way that the terminal transmits on the RACH control channel a connection set-up request to the access point. In practice, this means that the terminal transmits in the time slot allocated to the RACH control channel and at the same time the access point listens to the communication on the channel i.e. receives signals on the channel frequency it is using. When the access point detects that a terminal is transmitting a connection set-up request message, it performs the procedures necessary for the connection set-up, such as resource allocation for the connection, if it is possible. In the resource allocation, the quality of service requested for the connection is taken into account, which affects e.g. the number of time slots to be allocated for the connection. The access point informs the terminal whether the connection set-up is possible or not. If the connection set-up is successful, the access point transmits in the BCCH control field e.g. data on the transmission time slots, reception time slots, connection identifier, etc. which are allocated for the connection. The number of transmission and reception time slots is not necessarily the same, because in several cases the quantity of information to be transmitted is not the same in both directions. For example when using an Internet browser, the quantity of information transmitted from the terminal is considerably smaller than the amount of information received in the terminal. Thus, with respect to the terminal, the required number of transmission time slots is smaller than that of reception time slots. Furthermore, the number of time slots allocated for the connection can advantageously vary in different frames according to the need to transmit data. The access point controller is provided with a so-called scheduler, one function of which is the aforementioned allocation of time slots for different connections. The scheduler is implemented advantageously as an application program in the access point controller.
Since duplex data transmission is necessary in local area networks, duplex data transmission is also necessary on the radio channel. In a time division system this can be implemented either in such a way that some of the time slots of the frame are allocated for transmission from the wireless terminal to the access point (uplink) and some of them are allocated for the transmission from the access point to the wireless terminal (downlink), or in such a way that a separate frequency band is allocated for each data transmission direction. The HIPERLAN/2 system suggests the use of the former of the aforementioned methods, wherein the access point and the wireless terminals coupled to it do not transmit simultaneously.
In the HIPERLAN/2 systems, the access points can select the channel to be used in the connection irrespective of the other access points. Furthermore, the scheduler of the access point selects the moment of time to be used for the transmission irrespective of the other access points. In practice, this means that two or more access points can make an attempt to transmit simultaneously on the same channel, wherein the transmission is unsuccessful. In order to prevent this collision of transmissions, the transmitting access point or wireless terminal first listens to the signals of one or more frequencies. The access point selects a frequency with as little interference as possible, and if the interference level later rises at the selected frequency, the access point may e.g. change over to use another frequency.
As already presented above in this description, the radio local area network can comprise several access points, and the width of their coverage area is affected by e.g. the transmission level, ambient conditions, possible obstacles on the path of the signal, the directional pattern of the antenna, etc. In practice, it is not possible to determine the limits of the coverage area of access points clearly, but the coverage areas of access points located close to each other overlap at least partly. Thus, a wireless terminal may, in some cases, be in the coverage area of two or more access points, but these access points do not necessarily detect the transmissions of each other. Of these access points, the wireless terminal selects one for use in a data transmission connection. On the other hand, the access point to be used in the connection may be changed when the wireless terminal is moving or when the quality of the connection varies, which is known as such. Because the access points can select the channel to be used in the connection irrespective of other access points and time the transmissions independently, the properties of the data transmission between the wireless terminal and the access point to be used in the connection each time may be affected by another or several other access points, in whose coverage area the wireless terminal is located. Such interference may also occur in situations, in which the frequencies to be used are not the same, but two access points transmit e.g. on adjacent channels.
Access points which interfere with communication may also be access points of another radio network or other radio stations, radar stations, etc. This is possible particularly when there are several such radio systems within the same geographical area, which use at least partly overlapping frequency ranges or frequency ranges close to each other.
The properties of the data transmission are affected by not only interference by other radio devices but also changes in ambient conditions. These changes may be caused by e.g. multipath propagation of the signal, the moving of the wireless terminal within the operating range of the communication network, from the area of one cell to the area of another cell, or outside the operating range of the communication network, wherein the propagation conditions of the signal may vary. Also changes in air temperature and humidity may affect signal propagation and cause changes in the data transmission connection.
Upon setting up a data transmission connection, the wireless terminal listens, which access points transmit signals that can be received. The wireless terminal preferably also measures the signal strengths and selects e.g. the access point whose signal is the strongest at the moment. After this, the wireless terminal and the access point conduct connection set-up signalling, to transmit for example the parameters to be used in the connection, such as the required data transmission rate, the connection type, the communication channel, the time slots, as well as the connection identifier.
The wireless terminal typically also measures the strength of the signal of the access point to be used in the connection, as well as the strengths of the signals of possible other access points in the coverage area. If another access point is detected to have a sufficiently greater signal strength than the signal strength of the access point used at the moment, it is possible to hand the connection over to this second access point, which is known as such.
The access point can also request the wireless terminal to measure signal strengths. These measuring results are transmitted to the access point, which may, on the basis of the measuring results, analyze the interference level, find out about the cause of the interference e.g. whether the measured signal is a signal of a wireless terminal or access point belonging to the same radio local area network, a signal of an access point belonging to another radio local area network, or a signal caused by another system. As a result of the analysis of the measurements, the access point may e.g. perform a channel change, if it is expected that the quality of the connection is improved at the new channel frequency. The access point tries to identify the cause of the interference. If the access point detects that the interfering signal is a signal of a device belonging to a similar communication system but a different radio local area network (e.g. the local area network of another operator), the access point may try to reduce the effect of the interference e.g. by increasing the transmission level, by changing the channel coding, and/or by using modulation which is more resistant to interference. If the source of interference does not belong to the same system, the access point may try to reduce the interference preferably by changing the channel frequency. In HIPERLAN/2 systems and in other radio communication systems, in which the use of channel frequencies has not been taken into account at the planning stage, the access point should select the channel frequencies so that the available channel frequencies were used as evenly as possible and that the devices belonging to the same system would not disturb each other.
Recently, so-called intelligent antennas have been developed. They can be used e.g. as transmitting/receiving antennas of access points. Also for wireless terminals, such antenna solutions based on the intelligent antenna are under development. A significant property of the intelligent antenna is the adaptive tuning of the directional pattern of the antenna. Thus, it is possible in the access point to monitor moving terminals and try to direct the directional pattern of the antenna towards the terminal. In a corresponding manner, the wireless terminal may try to find out in which direction the access point is located and to direct the directional pattern of the antenna towards this direction. By means of this arrangement, the reception of the signal can be improved, wherein the range can be increased and/or the transmission level at the access point/terminal can be reduced. Furthermore, interference caused by radio signals with other radio devices and terminals/access points of the same radio local area network may decrease.
The use of the intelligent antenna may, however, cause errors in interpreting measurement results in prior art radio local area networks and other communication systems based on radio communication, in which radio interference is measured. This is due to the fact that in systems of prior art, it is not possible to take into account the effect of the intelligent antenna in the analysis of the measurement results, but the analysis is normally made presuming that the antenna is ommidirectional. Thus, the above-mentioned measurement results do not necessarily give a true reflection on the interference situation, because in the case of an omnidirectional antenna, the aim is to make the directional pattern substantially identical in all directions. As a result of the analysis, the access point may unnecessarily e.g. raise the transmission level or change the channel frequency, if the antenna used in the measurements does not comply with the assumption. This, in turn, may result in an increase, instead of decrease, of the interference level.
It is an aim of the present invention to provide a system, in which more reliable analyses of measurement results are obtained as compared with systems of prior art, wherein also the interference level can be reduced. One embodiment of a method, in a wireless communication system (1) which comprises wireless terminals (MT1-MT4) and at least one access point (AP1, AP2) and access point controller (APC1, APC2), includes the following: at least one antenna configuration is determined for an antenna (30) of the wireless terminal (MT1-MT4), the antenna (30) of the wireless terminal (MT1-MT4) is used for receiving a radio signal, the strength of the radio signal received by the antenna (30) of the wireless terminal (MT1-MT4) is measured, a measurement message (HD1, D1) is formed of one or more measurements, and said measurement message (HD1, D1) is transmitted from the wireless terminal (MT1-MT4) to the access point (AP1, AP2), characterized in that in the method, data (UAC) about the antenna configuration during the measurement is also added into said measurement message (HD1, D1).
In one aspect, the present invention is directed to a wireless communication system. In one embodiment, the wireless communication system comprises a wireless communication system (1) which comprises wireless terminals (MT1-MT4) comprising at least one antenna (3) for receiving radio signals, at least one antenna configuration being determined for the antenna (30), at least one access point (AP1, AP2) and access point controller (APC1, APC2), means (29) for measuring the strength of a radio signal received with the antenna (30) of the wireless terminal (MT1-MT4), means (11) for forming a measurement message (HD1, D1) of one or more measurements, and means (COM) for transmitting said measurement message (HD1, D1) from the wireless terminal (MT1-MT4) to the access point (AP1, AP2), characterized in that the wireless communication system (1) also comprises means (11) for adding data (UAC) indicating at least one antenna configuration used during the measurement into said measurement message (HD1, D1).
The wireless terminal according to another embodiment of the present invention comprises a wireless terminal (MT1-MT4) comprising at least one antenna (30) for receiving radio signals, at least one antenna configuration being determined for the antenna (30), means (29) for measuring the strength of a radio signal received with the antenna (30) of the wireless terminal (MT1-MT4), means (11) for forming a measurement message (HD1, D1) of one or several measurements, and means (COM) for transmitting said measurement message (HD1, D1), characterized in that the wireless terminal (MT1-MT4) also comprises means (11) for adding data (UAC) indicating at least one antenna configuration used during a measurement into said measurement message (HD1, D1).
In another aspect, the present invention is directed to an access point. In one embodiment, the access point comprises means (COM) for receiving a measurement message (HD1, D1) formed in the wireless terminal (MT1-MT4), characterized in that the access point (AP1, AP2) determines, from said measurement message, data (UAC) indicating at least one antenna configuration used during a measurement (HD1, D1).
The invention is based on the idea that in connection with the measurement results, it is possible to give the antenna type, wherein the effect of the antenna on the measurement results, i.e. deviations of the directional pattern from the default value, can be taken into account in the analysis of the measuring results. In the HIPERLAN/2 system, the access point is the unit in which the analysis is made.
The present invention gives significant advantages to methods and wireless communication systems of prior art. Using the method of the invention, the analysis of the measurement results can be made more reliable than is possible to achieve in systems of prior art. The invention can also be used to reduce the need for handover. In the communication system according to the invention, it is still possible to reduce the interference, because the access points do not increase their transmission level unnecessarily, wherein the utilization rate of the communication system is improved significantly.